This invention concerns a breathing assistance apparatus capable of operating in alternating inhalation and expiratory phases and comprising:                a pressurized respiratory gas source,        control means capable of transmitting a reference value of a gas related parameter to the said gas source,        an inhalation duct to supply the gas from the gas source to a patient,        an expiratory duct for the expiratory gas of the patient,        a valve on the inhalation duct, said valve comprising means that allow the gas to pass to make possible proportional operation, said valve being controlled by means which are distinct from the pressurized gas source,        a valve on the expiratory gas source to help establish a PEP,        sensors, respectively pressure and flow, on the inhalation duct.        
It is pointed out that the PEP (Positive Expiratory Pressure) will be defined below.
Apparatus of the type mentioned above are already known.
For example, a description of such an apparatus can be found in the document FR 2 812 203 (see FIG. 15 of this document for example).
FIG. 1 shows a diagrammatic representation of a known apparatus 10.
The apparatus 10 comprises a source 100 of pressurized respiratory gas connected to a first end of an inhalation duct 110, whose second end is in contact with a patient to supply him/her with gas from the source 100 during the inhalation phases.
This second end is shown here by a breathing mask 120. It can also be envisaged to use an apparatus of this type in “invasive” mode, the patient then is intubated with this second end of the duct.
The mask may have vents for leaks from the breathing mask.
The apparatus 10 also comprises an expiratory duct 130, whose first end is open to free air to evacuate the gases exhaled by the patient, and whose second end is connected to the patient, joining the second end of the inhalation duct.
Two sensors can be seen on the inhalation duct 110:                a respiratory gas pressure sensor 111 in the duct,        a flow rate sensor 112 for this same gas.        
Valves are fitted in order to close the respective ducts 110 and 130 selectively.
A valve 113 is thus placed on the duct 110, and a valve 133 on the duct 130.
The valve 113 that is positioned on the inhalation duct is the type of valve whose means for allowing the gas to pass comprise at least one part permitting proportional operation.
Typically, this concerns a valve in which the gas passage means include a cock that can be moved in rotation.
The operation of such a valve capable of operating proportionally corresponds to a specific configuration (proposed in the document FR 2 812 203) that is advantageous. We will come back to this aspect.
The valve 133 that is positioned on the expiratory duct is a different type of valve.
This valve comprises an inflatable bladder type sealing element capable of blocking the duct on which the valve is positioned and is pneumatically controlled.
The valve 113 is controlled by a dedicated electric motor (not shown).
The valve 133 is pneumatically controlled.
More precisely, the valve 133 can be selectively controlled by two different pressure lines, as this valve is connected to pressure line selection means which control the valve.
These two pressure lines are:                a line 1331 connected to the pressure source 100,        a line 1332 connected to an auxiliary pressure source 140.        
The apparatus 10 also comprises an assembly collectively designated under the reference 50, which corresponds to means for controlling the operation of the apparatus.
These control means 50 are capable of defining a reference operating value for the gas source 100, and transmitting it by means of a connection 1510.
This reference operating value is for example expressed in a number of revolutions per minute, in the case of the pressure source 100 being a turbine. It may also be a different type of reference value, for example a pressure output reference value from the pressurized gas source.
The reference value is a real time reference value and is constantly adapted by the control means 50.
Such an apparatus operates according to an alternating rhythm of:                inhalation phases when the valve 113 is in the open position and the valve 133 is in the closed position.        expiratory phases when the valve 113 is in the closed position and the valve 133 is:                    either in the open position,            or in the closed position so as to establish a positive counter pressure in the expiratory duct (PEP), which is designed to balance the residual excess pressure in the patient's lungs when the said patient is in the expiratory phase. In this case, a specific control for sealing the valve 133 is provided.                        
The reference value defined by the control means 50 to be transmitted to the pressurized gas source 100 is composed according to a parameter that is representative of the operation of the apparatus. We will come back to this aspect as part of the description of the invention.
Also, according to the nature of the parameter that is representative of the operation of the apparatus that is used to define this reference value for the gas source, the apparatus may be operated in different operating modes.
In particular, this apparatus can operate in barometric or volumetric mode.
In barometric mode, the apparatus is controlled according to the pressure in the inhalation duct. In this operating mode, the objective is to provide the patient with a desired pressure during each inhalation phase (see FR 2 812 203 page 19).
In this operating mode, it is a pressure parameter that is consequently used to define the reference value for the source 100. The flow rate in the inhalation duct is a value which results from the pressure control.
In volumetric mode, the apparatus is controlled according to the flow rate in the inhalation duct. In this operating mode, the objective is to provide the patient with a desired volume of gas during each inhalation phase (see FR 2 812 203 page 18).
More precisely, in the volumetric mode, the apparatus of FR 2 812 203 is controlled as follows to provide the patient with a desired volume of gas.
The apparatus of FR 2 812 203 comprises an inhalation valve with a rotating element whose rotation is controlled so as to form in the inhalation valve a passageway section having a dimension which corresponds to the volume which is desired for the patient.
The angular position of the rotating element of the inhalation valve is thus set at a value corresponding to the volume which is to be delivered to the patient.
Once a value of the desired volume to be delivered to the patient has been selected, this rotating element is brought into an angular position which corresponds to said value.
Furthermore, in the volumetric mode of the apparatus of FR 2 812 203 the pressure difference between the upstream and downstream parts of the inhalation valve is kept constant through the control of the pressure at the outlet of the gas source of the apparatus.
Thus, in the volumetric mode the apparatus of FR 2 812 203 undergoes two controls:                (a) control of the volume of gas delivered to the patient. This is achieved through the control of the angular position of the rotating element of the inhalation valve. For this purpose the angular position of said rotating element is brought to a position which defines in the inhalation valve a passageway which corresponds to the desired gas volume,        (b) control of the difference between the upstream and downstream parts of the inhalation valve. For this purpose the turbine is controlled by a control circuit of the apparatus. This pressure difference is controlled so as to keep a constant value, whatever the desired volume (or flow) for the patient.        
Keeping the pressure difference between the upstream and the downstream parts of the inhalation valve at a constant level (e.g., 10 mbars) allows having an inhalation valve which operates in a linear manner, e.g. the dimension of the passageway defined by the aperture of the rotating element shall be proportional to the flow going through the valve.
Coming back to the general features of the apparatus of FR 2 0812 203, the means for controlling the apparatus are provided to adapt simultaneously:                the operating mode (barometric or volumetric). To this end, it is possible to provide for manual selection of the operating mode desired, by an operator commanding an interface dedicated to this task (the keyboard of the apparatus for example).        and the value of the parameter that is representative of the operation of the apparatus that is used to define the reference value for the gas source, depending on:                    the operating mode used (barometric or volumetric, which determines in particular the nature of the parameter to be used) and,            the phase in which the apparatus is situated (inhalation or expiratory phase).                        
It is pointed out that if a PEP is to be used, the pneumatic pressure used to inflate the expiratory valve 133 bladder (control pressure) must be controlled with precision.
In fact, when a PEP is to be established in the mask 120 (or more generally at the patient, this text considering as equal the configuration with the mask and the invasive configuration, whether we are considering the presentation of the state of the technique or in that of the invention), the valve 133 must not be sealed by an excessive control pressure, but only by a counter pressure suitable for creating a PEP desired at the patient's level.
This is the reason why two control lines are provided for this valve 133:                the line 1331 allows the valve 133 to be sealed without having to worry about any balancing with a counter pressure in the ducts—especially during inhalation phases,        the line 1332, connected to the compressor, 140, permits a calibrated control pressure to be transmitted to the valve 133 in order to establish a desired PEP during the patient's exhalation during the expiratory phases.        
It is pointed out that the valve 133 is connected to selection means (not shown) to select the line 1331 or the line 1332.
The known apparatus of FIG. 1 is advantageous.
In particular, the inhalation valve of such a apparatus permits precise control of the respiratory gas in the inhalation duct—including for managing different operating modes.
It can however be envisaged to improve such an apparatus event 25 further.
Firstly, it would be advantageous to have means for controlling the apparatus that allow its operation to be controlled precisely, in its various operating modes.
In particular, when operating in volumetric mode, it is possible that very precise control of the flow rate value in the duct 110 is difficult.
This is especially the case when the pressure source is a turbine.
In this case indeed, the variations in load of the turbine are likely in certain conditions to disrupt the precision of the control of the respiratory gas flow in the duct 110.
This aspect is further strengthened in the case where the inhalation gas flow rate is to be controlled over wide ranges of possible values—for example from 1 to 180 litres/minute.
Such a range of flow rates may be desired, to allow pathologies and illnesses of different types to be treated.
Secondly, it can be difficult to minimize the base flow rate.
In general, it is preferred to minimize this base flow rate during the expiratory phases, in particular to avoid wasting secondary gases such as oxygen which can be mixed in the gas issued from the source 100.
It is therefore desired that a base flow rate whose value just corresponds to the leaks of the apparatus (for example leaks in the mask 120).
This aspect in a way brings us back to the precise control of a flow rate, especially for very low reference values.